Refrigerant system with intercooler and liquid/vapor injection

ABSTRACT

A refrigerant system is provided with at least two sequential stages of compression. An intercooler is positioned intermediate the two stages. The refrigerant flowing through the intercooler is cooled by a secondary fluid such as ambient air. A vapor/liquid injection function is also provided for the refrigerant system. The intercooler function and the vapor/liquid injection function are selectively activated on demand depending on environmental conditions and thermal load in a conditioned space. This invention is particularly important for the CO2 refrigerant systems operating in the transcritical cycle.

BACKGROUND OF THE INVENTION

This application relates to refrigerant systems, wherein the compressoris a multi-stage compressor (e.g. a two-stage compressor), and whereinan intercooler and liquid/vapor injection are provided between thecompression stages. The intercooler is preferably subjected to anambient airflow and, such that the cooling in the intercooler ispreferably provided by circuitry and components that are already part ofthe refrigerant system.

Air conditioning, heat pump and refrigeration systems provide cooling orheating of a secondary fluid, such as air, delivered into aclimate-controlled environment. A typical basic air conditioning, heatpump or refrigeration system includes a compressor, an expansion device,a heat rejecting heat exchanger and a heat accepting heat exchanger. Theheat rejecting heat exchanger is either a condenser for subcriticalapplications or a gas cooler for transcritical applications, while aheat accepting heat exchanger is typically an evaporator. The heat pumpsalso include a refrigerant flow reversing device, typically a four-wayvalve that allows for refrigerant flow reversals throughout therefrigerant system while switching between cooling and heating modes ofoperation.

To obtain additional capacity, enhance system efficiency and achievehigher compression ratios without exceeding the discharge temperaturethreshold, it is often the case that a two-stage compressor (or athree-stage compressor, in some cases) is provided in a refrigerantsystem. With a two-stage compressor, two separate compression members ortwo separate compressor units are disposed in series. Specifically, forinstance, in the case of a reciprocating compressor, two separatecompression members may be represented by different banks of cylindersconnected in series. Refrigerant compressed by a lower stage to anintermediate pressure is delivered from a discharge outlet of this lowerstage to the suction inlet of the upper stage. If the compression ratiofor the compressor system is high (which is typically the case fortwo-stage compression systems) and/or refrigerant suction temperature ishigh (which is often the case for a refrigerant system equipped with aliquid-suction heat exchanger), then refrigerant discharge temperaturecan also become extremely high, and in many cases may exceed the limitdefined by the safety or reliability considerations.

Thus, it is known in the art to provide an intercooler heat exchanger(or a so-called intercooler) between the compression stages to extendthe operational envelope and/or improve system performance andreliability. In an intercooler, refrigerant flowing between the twocompression stages is typically cooled by a secondary fluid. Quiteoften, additional components and circuitry are required to providecooling of the refrigerant in the intercooler. As an example, a fan orpump is included to move a secondary cooling fluid from a coldtemperature source to cool the refrigerant in the intercooler.

It is also known in the art to provide refrigerant liquid/vaporinjection to reduce discharge temperature, extend the compressoroperational envelope and improve system performance and reliability. Insuch refrigerant systems, at least a portion of refrigerant leaving aheat rejecting heat exchanger is partially expanded in an auxiliaryexpansion device to an intermediate pressure and temperature and routedto a point between the compression stages where it is mixed with therefrigerant partially compressed in a lower compression stage and to bedelivered to an upper compression stage. As also known, the vaporinjection circuit may include an economizer heat exchanger to provideadditional cooling to the refrigerant circulating through the maincircuit and thus provide additional capacity to the refrigerant system.

Recently, new generation refrigerants, such as natural refrigerants, arebeing utilized in refrigerant systems. One very promising refrigerant iscarbon dioxide (also known as CO₂ or R744). Particularly with CO₂refrigerant systems, an intercooler and refrigerant liquid/vaporinjection functions become even more important, as these refrigerantsystems tend to operate at high discharge temperatures due to highoperating pressures, use of a liquid-suction heat exchanger, a highvalue of the polytropic compression exponent for the CO₂ refrigerantand, in general, by the transcritical nature of the CO₂ cycle. However,the additional cost of the circuitry and components associated with theintercooler and liquid/vapor injection, along with the limited benefitsfor prior art refrigerant systems utilizing conventional refrigerants,made the provision of an intercooler and liquid/vapor injection in theconventional refrigerant systems less practical.

Thus, it is desirable to provide an intercooler and liquid/vaporinjection for a multi-stage compressor refrigerant system, andparticularly for a CO₂ refrigerant system, as well as a selectiveactivation method of these components to achieve the most efficient andreliable operation of a refrigerant system over a wider spectrum ofenvironmental conditions.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, a refrigerant systemincorporates a multi-stage compressor. An intercooler and liquid/vaporinjection are provided between at least two of the compression stagesconnected in series. The intercooler is preferably positioned to besubjected to an airflow passing over a heat rejecting heat exchanger. Inone configuration, an intercooler is positioned in series with the heatrejecting heat exchanger, with respect to the ambient airflow, and inanother configuration, an intercooler is positioned in parallel with theheat rejecting heat exchanger, with respect to the ambient airflow.Further, an outdoor fan that passes air over the heat rejecting heatexchanger may also provide cooling for the intercooler, while both heatexchangers may or may not share the same construction.

In one arrangement, an intercooler is positioned between the samecompression stages where a liquid/vapor injection function is provided,and in another arrangement, an intercooler is positioned betweendifferent compression stages than the compression stages between whichliquid/vapor injection function is provided.

At certain environmental conditions and thermal load demands, anintercooler may be engaged at the same time when liquid/vapor injectionis activated. On the other hand, at other environmental conditions andthermal load demands, either an intercooler or liquid/vapor injectionfunction may be more preferable.

The intercooler increases system capacity and improves efficiency, sincethe compressor discharge temperature is reduced, and the heat rejectingheat exchanger is typically capable to cool refrigerant to a lowertemperature, providing a higher cooling potential in the evaporator.Additionally, a steeper slope of the isentropic lines for the downstreamcompression stages allows for a higher compressor isentropic efficiency.Furthermore, lower discharge temperatures promote higher compressorreliability and operational envelope extension.

Additionally, if the refrigerant system operates in a transcriticalcycle, where high side temperature and pressure are independent fromeach other, the discharge pressure is no longer limited by a dischargetemperature and can be adjusted to a specified value for an optimumperformance level. Thus, the transcritical refrigerant system efficiencyand capacity are enhanced even further.

Liquid/vapor injection provides similar benefits but may be activated atdifferent environmental conditions and thermal load demands.Additionally, in case an economizer heat exchanger is provided, extrasubcooling and additional thermal potential are gained in theevaporator.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an inventive refrigerant system.

FIG. 2 shows a second schematic of an inventive refrigerant system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 20 is illustrated in FIG. 1 having a lower stagecompressor 22 and a higher stage compressor 24. While only twosequential stages are shown, additional stages may also be incorporatedin series in this invention. Also, instead of separate compressorsconnected in sequence, a multi-stage single compressor arrangement canbe employed and equally benefit from the present invention. Forinstance, the two illustrated, separate compression members may berepresented by different banks of cylinders connected in series for areciprocating compressor. As known, refrigerant compressed by a lowerstage compressor 22 to an intermediate pressure is delivered from adischarge outlet of this lower stage compressor 22 to the suction inletof the higher stage compressor 24. An intercooler 26 is positionedbetween the two stages to accept refrigerant from a discharge outlet ofthe lower stage compressor 22. This refrigerant is cooled by a secondarymedia, such as ambient air blowing over external heat transfer surfacesof the intercooler 26, during heat transfer interaction with therefrigerant, is delivered downstream to a suction inlet of the higherstage compressor 24. Again, if additional stages of compression areprovided, additional intercoolers may also be positioned between thosestages.

Further, an intercooler bypass line 28 incorporating a refrigerant flowcontrol device 25 may be provided. An intercooler bypass line bypassesat least a portion of refrigerant around the intercooler 26 when fullintercooling capability may not be required. A refrigerant flow controldevice 25 may be, for instance, a fixed restriction orifice, on/off orpulsing solenoid valve or a modulating valve. The last two refrigerantflow control devices provide regulating capability for the amount ofrefrigerant bypassing the intercooler 26. In case extra refrigerant flowcontrol flexibility may be needed, an additional refrigerant flowcontrol device 23 may be positioned within intercooler circuit tocontrol refrigerant flow through the intercooler 26. The refrigerantflow control device 23 may be of an on/off or pulsing solenoid valvetype or a modulating valve type. Further, the independent refrigerantflow control devices 23 and 25 may be combined into a three-way valve ofa regular on/off type or a regulating type.

A fan or other air-moving device 34 moves air over a heat rejecting heatexchanger 30 and the intercooler 26. In cases when a separate air-movingdevice is implemented to blow air over external surfaces of theintercooler 26, this air-moving device may be driven by a variable speedmotor or a multi-speed motor to provide additional flexibility in theintercooler operation and control.

The intercooler 26 may be positioned within the same structure as theheat rejecting heat exchanger 30 or may be positioned to comprise itsown structure. If the intercooler 26 shares the same structure with theheat rejecting heat exchanger 30, the two heat exchangers may bepositioned in a parallel configuration or in a serial configuration,with respect to the airflow. In the latter case, the intercooler 26 ispreferably positioned upstream of the heat rejecting heat exchanger 30,in relation to the airflow, and such that the fan 34 also moves air overthe external surfaces of the intercooler 26. Also, as mentioned above,the intercooler 26 may have its own fan. In the case of the intercooler26 position upstream of the heat rejection heat exchanger 30, althoughthe air stream will be preheated by the intercooler 26 before reachingthe heat rejecting heat exchanger 30, during heat transfer interactionbetween the air and refrigerant in the intercooler 26, the temperatureof the refrigerant flowing through the intercooler 26 is reduced, asdesired, as well as the refrigerant system 20 will have a more compactdesign. As also known, other secondary media such as water or glycol canbe used instead of air, and consequently, the fan 34 can be replaced bya liquid pump circulating this fluid through a secondary circuit.

As is also known, an expansion device 40 is positioned between the heatrejecting heat exchanger 30 and an evaporator 32 with associatedair-moving device such as fan 36 blowing air over external surfaces ofthe evaporator 32.

The intercooler 26 extends an operational envelope of the refrigerantsystem 20, as well as increases its capacity and efficiency, since thecompressor discharge temperature is reduced and the heat rejecting heatexchanger 30 may be capable to cool refrigerant to a lower temperature,providing a higher cooling potential for the refrigerant entering theevaporator 32. Compressor power consumption may also be reduced, as heatremoved from the compression process is rejected at the lower high sidepressure. Also, a steeper slope of the isentropic lines for thedownstream compression stages allows for a higher compressor isentropicefficiency. Additionally, if the refrigerant system 20 operates in atranscritical cycle, where the high side temperature and pressure areindependent from each other, the discharge pressure is not limited by adischarge temperature anymore and can be adjusted to a valuecorresponding to an optimum performance level. Furthermore, in bothsubcritical and transcritical cycles, the temperature of the refrigerantdischarged from the higher compression stage 24 is reduced, improvingreliability of the compressor. Thus, performance (efficiency andcapacity) of the refrigerant system 20 is increased and compressorreliability is improved.

The refrigerant system 20 also includes a vapor/liquid injection line 27that incorporates an auxiliary expansion device 29. When thevapor/liquid injection circuit is activated, at least a portion ofrefrigerant exiting heat rejecting heat exchanger 30 is rerouted throughthe vapor/liquid injection line 27 to be expanded to a lower pressureand temperature in the auxiliary expansion device 29 and injected inbetween the lower and upper compression stages 22 and 24. Since thisportion of refrigerant has a lower temperature it can cool partiallycompressed main refrigerant to subsequently achieve a lower dischargetemperature. It should be pointed out that in case the auxiliaryexpansion device 29 is not equipped with the shutoff functionality, anadditional shutoff valve may be required in the vapor/liquid injectionline 27. The vapor/liquid injection line 27 may contain a liquid-vaporrefrigerant mixture, if the end state for the expansion process in theauxiliary expansion device 29 is located inside the two-phase dome, ormay contain purely liquid refrigerant, if the end state for theexpansion process in the auxiliary expansion device 29 is still locatedoutside of the two-phase dome. This would depend on the refrigerant typeas well as environmental and operating conditions. The injection pointis preferably positioned downstream of the intercooler 26 and upstreamof the second compression stage 24.

Therefore, the refrigerant system 20 can utilize either the intercooler26, vapor/liquid injection through the injection line 27 orsimultaneously both of these functions to reduce discharge temperatureand achieve all the benefits outlined hereinabove. Which function is tobe activated will depend on environmental and operating conditions, aswill be explained below.

FIG. 2 shows another embodiment 120, wherein a refrigerant system hasthree sequential compression stages 122, 122A and 124. A refrigerantconnection line 126 intermediate higher compression stages 122A and 124is routed to be in the path of air being flown over the heat rejectingheat exchanger 130 by a an associated fan 134. As shown, the refrigerantconnection line 126 may or may not have a heat transfer enhancementstructure 156 and performs an intercooling function, as discussed inreference to the FIG. 1 embodiment. A bypass line 128 bypasses at leasta portion of refrigerant around the intercooling line 126, if desired,and as in the FIG. 1 embodiment includes a refrigerant flow controldevice 125. An expansion device 140, an evaporator 132 with anassociated fan 136, a vapor/liquid injection line 127 incorporating anauxiliary expansion device 129 are included and similar to the FIG. 1embodiment. Additionally, an economizer heat exchanger 144 is positioneddownstream of the heat rejection heat exchanger 130, with respect torefrigerant flow. When an economizer circuit is activated, a portion ofrefrigerant is expanded to a lower pressure in an economizer expansiondevice 142 and diverted via an economizer line 138 to a point betweencompression stages 122 and 122A. Since this economized refrigerant is atcolder temperature than the main refrigerant exiting the heat rejectingheat exchanger 130, it can cool this main refrigerant, during heattransfer interaction in the economizer heat exchanger 144, enhancingrefrigerant system 120 performance characteristics (capacity andefficiency). Further, this economized refrigerant can cool partiallycompressed refrigerant by the lower compression stage 122, while mixingwith this refrigerant. In case the economizer expansion device 142 isnot equipped with the shutoff capability, an additional shutoff valvemay be required for the economizer circuit. As known, an economizercircuit can have a number of different configurations including, but notlimited to, arrangements for tapping an economized refrigerant flowupstream and downstream of the economizer heat exchanger 144, as well asschematics incorporating a flash tank.

The refrigerant system 120 can utilize either the intercooling line 126,vapor/liquid injection through the injection line 127, economizerfunction through the economizer line 138 or any combination of thesefunctions to reduce discharge temperature and achieve all the benefitsoutlined hereinabove. Which function is to be activated will depend onenvironmental and operating conditions, as will be explained below.

The present invention is particularly useful in refrigerant systems thatutilize CO₂ as a refrigerant, since the CO₂ refrigerant has a high valueof a polytropic compression exponent, and high side operating pressuresand pressure ratios of such systems can be very high, promoting higherthan normal discharge temperatures. Still, the invention would extend torefrigerant systems utilizing other refrigerants.

When augmented system capacity is required by thermal load demands inthe conditioned space or/and by high ambient temperature—low indoortemperature environmental conditions and the compressor dischargetemperature needs to be reduced at the same time, an economizer functionis turned on (if present), a vapor/liquid injection function is turnedoff and an intercooler function may be turned on (especially fortranscritical applications). The economizer line typically returnsrefrigerant between lower compression stages to achieve maximumtemperature difference in the economizer heat exchanger and maximumcapacity boost, and by the time the refrigerant reaches the highercompression stages, it may need to be additionally cooled to eithersatisfy the discharge temperature requirements or provide decoupling forpressure and temperature in transcritical applications. The intercooleris typically provided between the higher compression stages, since therefrigerant in the intercooler needs to be at a noticeably highertemperature than the cooling media such as ambient air, in order toprovide positive intercooling effect. If the economizer and intercoolerare positioned between the same compression stages, then the economizerwould be preferably positioned upstream of the intercooler, for thereasons outlined above. The vapor/liquid injection function is turnedoff to provide maximum refrigerant flow in the evaporator andsubsequently maximum capacity. In case the discharge temperature isstill above the predetermined threshold, the vapor/liquid injectionfunction would be activated. The vapor/liquid injection function may bepositioned in between the same compression stages as the intercoolerfunction or in between lower compression stages. The vapor/liquidinjection function could be switched to be redirected in betweendifferent compression stages as well, if desired.

If reduced capacity may be needed and lower discharge temperature issimultaneously required, then vapor/liquid injection is activated firstand is followed by the intercooler function engagement, if required. Incase of refrigerant system capacity matching thermal load demands in theconditioned space or system capacity reduction provided by otheravailable unloading options, the intercooler function is activated firstto approach the desired discharge temperature that is followed by thevapor/liquid injection as a second stage of the discharge temperaturereduction.

As stated hereinabove, the vapor/liquid injection function and theintercooler function could be adjusted via modulating or pulsing controltechniques for the refrigerant flow control devices such as valves. Forthe intercooler function, the adaptive control can be applied to theairflow passing over the intercooler external surfaces, for instance, bya variable speed or multi-speed air-moving device such as a fan.

It should be noted that this invention is not limited to the refrigerantsystems shown in the FIGS. 1 and 2, as the actual refrigerant system mayinclude additional components, such as, for example, a liquid-suctionheat exchanger, a reheat coil, an additional intercooler, an additionaleconomizer heat exchanger or a flash tank. The individual compressionstages may include several compressors arranged in tandem. Thecompressors can be of variable capacity type, including variable speedand multi-speed configurations. Further, the compressors may havevarious unloading options, including intermediate pressure to suctionpressure bypass arrangement, or the compressors may be unloadedinternally, as for example, by separating fixed and orbiting scrollsfrom each other on an intermittent basis. These system configurationsare also not limited to a particular compressor type and may includescroll compressors, screw compressors (single or multi-rotorconfigurations), reciprocating compressors (where, for example, some ofthe cylinders are used as a low compression stage and other cylindersare used as a high compression stage) and rotary compressors. Therefrigerant system may also consist of multiple separate circuits. Thepresent invention would also apply to a broad range of systems, forexample, including mobile container, truck-trailer and automotivesystems, packaged commercial rooftop units, supermarket installations,residential units, environmental control units, etc., as well as beextended to the heat pump applications.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. A refrigerant system comprising: a compressor assembly including atleast two stages of compression connected in series, with a lowercompression stage compressing refrigerant from a suction pressure to anintermediate pressure and passing this refrigerant to a highercompression stage compressing refrigerant from an intermediate pressureto a discharge pressure; an intercooler positioned intermediate of saidlower and higher compression stages; a liquid/vapor injection functionwith a vapor injection connection positioned intermediate of said lowerand higher compression stages; a heat rejecting heat exchangerpositioned downstream of said higher compression stage, an evaporatorpositioned upstream of said lower compression stage and an expansiondevice positioned intermediate of said heat rejecting heat exchanger andsaid evaporator; at least one secondary fluid moving device for movingsecondary fluid in at least one secondary fluid path over said heatrejecting heat exchanger and said intercooler; and said intercooler andsaid liquid/vapor injection function are selectively activated tocontrol refrigerant discharge temperature depending on environmental andoperational conditions as well as thermal load demands in a conditionedspace. 2.-6. (canceled)
 7. The refrigerant system as set forth in claim1, wherein said at least two compression stages are positioned withinone compressor.
 8. The refrigerant system as set forth in claim 1,wherein said at least two compression stages are represented by separatecompressors.
 9. The refrigerant system as set forth in claim 1, whereinthe refrigerant system operates at least in part in the transcriticalcycle.
 10. The refrigerant system as set forth in claim 1, wherein therefrigerant system operates at least in part in the subcritical cycle.11. The refrigerant system as set forth in claim 1, wherein saidliquid/vapor injection function includes an economizer heat exchanger ora flash tank.
 12. The refrigerant system as set forth in claim 1,wherein said at least two compression stages include three compressionstages.
 13. The refrigerant system as set forth in claim 12, whereinsaid intercooler and said liquid/vapor injection function are positionedbetween the same lower and higher compression stages.
 14. Therefrigerant system as set forth in claim 13, said liquid/vapor injectionfunction is positioned downstream of said intercooler, with respect torefrigerant flow.
 15. The refrigerant system as set forth in claim 12,wherein said intercooler and said liquid/vapor injection function arepositioned between different lower and higher compression stages. 16.The refrigerant system as set forth in claim 15, wherein saidintercooler is positioned between the higher compression stages and saidliquid/vapor injection function is positioned between lower compressionstages.
 17. The refrigerant system as set forth in claim 1, wherein saidrefrigerant system includes refrigerant bypass line around saidintercooler and said intercooler being at least partially disengaged ondemand.
 18. The refrigerant system as set forth in claim 17, whereinsaid refrigerant system has control capability to control refrigerantflow through the intercooler.
 19. The refrigerant system as set forth inclaim 1, wherein said intercooler has a separate secondary fluid movingdevice and said secondary fluid moving device has capability to vary aflow of secondary fluid.
 20. The refrigerant system as set forth inclaim 1, wherein said liquid/vapor injection function is equipped withan economizer heat exchanger and further wherein said economizedliquid/vapor injection function is engaged first, said intercooler isengaged second and said non-economized liquid/vapor injection functionis engaged third to control discharge temperature, if extra capacity isrequired to control environmental conditions in a climate-controlledspace.
 21. The refrigerant system as set forth in claim 1, wherein saidintercooler is engaged first and said liquid/vapor injection function isengaged second to control discharge temperature, if no extra capacity isrequired to control environmental conditions in a climate-controlledspace.
 22. The refrigerant system as set forth in claim 1, wherein saidliquid/vapor injection function is engaged first and said intercooler isengaged second to control discharge temperature, if reduced capacity isrequired to control environmental conditions in a climate-controlledspace.
 23. A method of operating a refrigerant system including thesteps of: (a) providing a compressor assembly including at least twostages of compression connected in series, with a lower compressionstage compressing refrigerant from a suction pressure to an intermediatepressure and passing this refrigerant to a higher compression stagecompressing refrigerant from an intermediate pressure to a dischargepressure; (b) positioning an intercooler intermediate of said lower andhigher compression stages; (c) positioning a liquid/vapor injectionfunction with a vapor injection connection intermediate of said lowerand higher compression stages; (d) positioning a heat rejecting heatexchanger downstream of said higher compression stage, positioning anevaporator upstream of said lower compression stage and positioning anexpansion device intermediate of said heat rejecting heat exchanger andsaid evaporator; (e) moving a secondary fluid in at least one secondaryfluid path over said heat rejecting heat exchanger and said intercooler;and (f) selectively activating said intercooler and said liquid/vaporinjection function to control refrigerant discharge temperaturedepending on environmental and operational conditions as well as thermalload demands in a conditioned space.
 24. The method as set forth inclaim 23, wherein said refrigerant system includes refrigerant bypassline around said intercooler and said intercooler being at leastpartially disengaged on demand.
 25. The method as set forth in claim 23,wherein said intercooler has a separate secondary fluid moving deviceand said secondary fluid moving device has capability to vary a flow ofsecondary fluid.